This invention relates to a method and apparatus for modifying a video signal so that a videotape recording of a video signal produces generally unacceptable pictures while a television monitor/receiver produces a generally normal picture from the modified signal.
There exists a need for a method and system for modifying a video signal so that the signal produces a normal color picture on a television receiver, but videotape recording of the video signal is inhibited or prevented.
Some of the early work resulted in methods which are not satisfactory. For example, U.S. Pat. No. 4,163,253 was issued to Morio et al. in July of 1979. This patent describes an arrangement in which a positive pulse is added to the back porch portion of each horizontal blanking interval. This approach results in significant playability problems. In this connection, video cassette recorders designed for the consumer market place invariably feature some form of automatic gain control circuitry (AGC). The AGC circuit insures that the video level applied to the FM modulator in the recording system remains at a fixed, predetermined value, even if the video level applied to the machine's input terminal varies widely about the nominal value. Without an AGC system, high level signals would be distorted and clipped and low level signals would be marred by the noise and interference products generated by the recording process. If the input level dropped to less than 1/3 of normal value, the replayed signal might not even be strong enough to reliably sychronize the receiver's timebases, hence giving rise to noisy unstable pictures. This early system, however, did not take into consideration the fact that many television monitor/receivers use the back portion region of a video signal for black-level clamping. The result is that while the approach described in the Morio et al. patent of confusing the AGC of a video cassette recorder does result in a copied tape being generally unviewable, the video signal also does not provide viewable pictures on many television monitor/receivers when it is used directly. This makes the process useless in practice.
The instant inventor is responsible for more recent and more successful approaches to preventing the unauthorized recording of a video signal. Two of such approaches are described in U.S. Pat. Nos. 4,577,216 and 4,631,603. The approach described and claimed in U.S. Pat. No. 4,631,603 and the approach described and claimed in continuing copending U.S. patent application Ser. No. 935,055 are similar to the Morio et al. arrangement, in that they rely on confusing the AGC of a video cassette recorder. They differ from the Morio et al. arrangement, however, in a major aspect. That is, they rely on differences between a video cassette recorder and a television receiver to assure that while the modifications made to a video signal prevent copying by a video cassette recorder, they do not significantly affect the playability of such signal on a conventional television monitor/receiver.